High-temperature coefficient resistor and method of making it



March W53 J. A. BECKER ETAL 2,633,521

HIGH-TEMPERATURE COEFFICIENT RESISTOR AND METHOD OF MAKING IT Filed June 28, 1949 2 SHEETS-SHEET 1 FIG. I

PREPARING PREPAR/NG RES/STANCE MATERIAL ELECTRODE MATERIAL SPREAD/N6 FILM OF ELECTRODE MATERIAL ON BACKING SPREAD/N6 FILM OF RES/STANCE MATERIAL OVER ELECTRODE MATERIAL SPREAD/N6 OF ELECTRODE MATERIAL OVER RESISTANCE AND EL EC T RODE MATERIAL ALREADY LAID DOWN CUTTING FILM INTO FLA/(ES PUTTING F LAKES 0N CARR/ER HEAT TREATING FLA/(ES F/GZ J. A. BEG/(1? INVENTORS H. CHRISTENSEN A r Tom/Er Man-ch 31, 1953 .1. A. BECKER HAL 2,633,521

HIGH-TEMPERATURE COEFFICIENT RESISTOR AND METHOD OF MAKING IT Filed June 28, 1949 2 Sl-IEETS-SI-IEET 2 FIG 4 J. A. BECKER H. CHRISTENSEN Byaw AT TQRNE V Patented Mar. 31, 1953 UNITED ST HIGH-TEMPERATURE COEFFICIENT RESIS- TOR AND METHOD OF MAKING IT Joseph A. Becker, Summit, and Howard Christensen, Springfield, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York,

N. Y., a corporation of New York Application June 28, 1949, Serial No. 101,778

12 Claims.

This invention relates to resistors and more particularly to small thin film resistors and to methods of making them.-

One general object of this invention is to improve the characteristics of and the production techniques for conductive devices having high temperature coeificients of resistance.

More specifically, objects of this invention are to lower the resistance, to increase the speed of thermal response, to eliminate noise, to increase the power capacity and to simplify the method of producing thermal sensitive resistance devices.

Conductive materials having high temperature coefiicients of resistance have been called thermistor materials and devices employing such materials in their conductive paths have been identified as "thermistors for convenience of terminology. Where these terms are employed hereinafter a material or device of the type indicated is intended.

Heretofore, resistors have been formed into thin films or flakes as disclosed, for example, in Patent 2,414,793 of January 28, 1947, issued to the present inventors. That device comprised a flake of resistance material produced by spreading a paste of a suitable material mixed in a fluid vehicle upon a drying surface, drying the paste to form a film, removing the dried film from the surface, cutting it into the desired shape and size, placing each bit or flake on a refractory surface and firing it to remove the binder to cure it so the material is completely sintered. The flake is supplied with electrodes.

This invention provides an improvement upon the method and devices discussed above. The electrodes are mounted on the flake in the steps of manufacture prior to the firing operation thereby effecting a considerable economy of labor.

Another feature is the application of electrodes which are in intimate association with the resistor material and therefore do not give rise to noise and modulation which often occur at the interface between semiconductive and conductive materials.

In accordance with this invention, green or uncured flake resistors are prepared with a green layer of electrode material on one or both of their major surfaces. The unit is fired to remove the binder in both the resistance and the electrode material and then both materials are sintered.

The above noted and other objects and features of this invention will be more fully understood from the following detailed description when read in conjunction With the accompanying drawings in which:

Fig. 1 represents a flow chart of the steps in the process of preparing resistor elements in ac cordance with this invention;

Fig. 2 shows one form of a carrier for support- 2 ing resistor elements during the heat treatment;

Fig. 3 is an enlarged cross-section of a por--'. tion of a flake after heat treatment;

Fig. 4 is a sectional view showing a portion of the flake of Fig. 3 mounted on a conductive backing for use in one form of resistor; and

Fig. 5 is a sectional view of another form of flake resistor embodying features of this invention.

As indicated in the flow chart of Fig. 1, the initial step in producing flake thermistors in accordance with this invention comprises preparing the electrode and resistance material employed. The materials are made up of a mixture of granular particles of the material which forms the effective portions of the film, a binder which can be removed without leaving any detrimental residues and a solvent for the'binder which gives it the fluid properties necessary to form a suitable vehicle for the finely divided particles of the effective portion.

Flake thermistors in accordance with this in? vention are preferably very thin, the thermistor layer being of a few microns to several tens of microns thickness and the electrode layers on top and bottom surfaces of the flake being of the order of from .5 to 2 microns. In order that the electrode and thermistor films are sufiiciently homogeneous to give the necessary uniformity of operation, the size of the particles which are used in making these films is of considerable importance. It has been found desirable in the preparation of the resistance and electrode material to employ a granular material having a maximum particle size about the desired thickness of the complete layer.

The effective thermistor material employed may be of various materials having relatively high temperature coefilcients of resistance. For example, one or more of the oxides of manganese, nickel, cobalt, copper, iron, or zinc may be used. This material is employed in finely divided granular form. The mixture employed in forming the resistor is made up in the form of a paste comprising a quantity of the prepared resistance material thoroughly mixed, as in a ball mill, with a temporary binder such as polymerized methacrylates (e. g., isobutyl methacrylate), polyvinyl buteral, polyvinyl chloride, or cellulose acetate butyrate and a volatile solvent. While a plasticizer such as dimethyl phthalate maybe added in the case of a polymerized methaerylate where desirable it is generally found unnecessary, the plasticity of the binder being controlled by the degree of its polymerization.

The electrode material is prepared in paste form by milling finely divided metal such as platinum black or palladium black or other noble metal with a binder in a solvent in the manner described for preparing the thermistor material.

For somezapplications" the quality of .the final thermistor is improved by employing an electrode mixture composed of thermistor material and a noble metal.

ture for the electrode, the formationrof'theusual barrier layer at the electrode-thermistor. inter-- face is reduced since there is a graduating of the conductivity from that of the 'the'rmistor .tozthe. high conductivity portion of the circuit thereby.

The thermistor material used in this mixture is the samec asrthat-usedforcthe reducing the noise andmodulation associated" with the barrier layer. In either type of contact mixture, the'metal-Eused must sinterat the-thermistor' material firing temperature and yet not evaporate, diifuse-into"the thermistor body. nor act asa'surface catalyst forbreaking down atmospheric oxygen which'might pass through the electrode'mate'rial to the b'ody-of the'flake-and changeitsresistance.

' While the binder-and solvent ofthe'thermis'tor and electrode material may be identical, it is often? desirable-to employ a binder and' solvent for onewhieh is mutually insoluble with that of the other so th'at theme will not soften-the other when theycomeinto contact prior to the drying and sintering' operation. Examples of suchbinders; suitable for use with the electrode material incombination with those binders previously mentioned, arecarbowax (poloxy ethylene) with water as a solvent and acopolymer butadine styrene with a' solvent of cyclohexane.

Informing a thermistor; avery thin film of electrode paste; prepared" in accordance with one ofthe alternatives heretofore set i forth; is spread out'on an'opticalflat'of glass. This may be done by means of a properly-spaced straight edge'or-by spraying so that upon drying the film willbe of the order of *to=2 microns in thick-' ness. This film is then driedin adust free atmosphere by passing air'at room temperature overit to-evaporateth'e volatile binder solvent. A filmof the thermistor material is then=spread on top of the'el'ectrode film so that on drying 'its thickness-willbe'on the order of tens of microns. When this film i's'dry,'a third thin filin' of electrode materialis smearedover-it' and dried; 1

The composite film is then removed-from the flat inthe form ofa sheet; This may'bedon'eby applyin'g an agent which causes a swelling of the temporary binderof the film; A bathv 'of'water has been found" satisfactory: for removing those films made with i the binders heretofore set forth which are not :watersoluble while some other swelling agent must "be" employed for those having the water soluble binderfsuggested. Thefilm can then be lifted away fr'om the fl'atiby means of some-thin member such asa razor-blade. An-

other way-of removingthe film'isto place -a-sheet of lint-free paper" moistened witha suitable swellingagent over the film 'for: a short time to wet it; then toremove the paper and lift the film with a thin-member as before."

' After-the" swelling agent evaporates the sheet is found to'=be-"quite *flexible and can be handled readily: It is out into-the desired shape and size of flakes, forexample; bya sharp blade-and then? fired. At this :point' one" oftlie advantages bf a; metal-thermistor material mixture for the electrode material"becomesrevident:' Inxthe cutting of'theflexibl'e composite filmit is sometimes foundthat small amounts-of electrode material spread' across" the edges; of; the flakes. thereby shunting the thermistonlayersz' While the metal-thermistor mixture material provides an excellent contact of low resistance over the face of. the-flake when. in small area paths such as would be formed across the edge of the thermistor material in this cutting operation its resistance ishigh enough to avoid having any material efiect'upon the electrical characteristics of the flake. The; dilution of the metal with thermistor material actually serves to prevent a:c.ontinuous path of electrical flow through contiguous metal particles.

These flakes are fired to remove the temporary binder and sinter the respective layer. Each flake isplaced on aifl'at surface of refractory-material; which. may 'b'e of" platinum coated with aluminum oxide or aluminum. oxide. alonemto avoid any tendency of the flakesatozstickrthereto during the firing operation. These: plates are of relatively large area withrespect to. that .iof the flakes in order-to insure uniformitemperaature distribution across the .plate; thereby .-'inhibiting any tendency of thefiake to" warp due to'uneven heating"; A plurality of thecarrier plates are stacked with spacers between-them in preparation for 'firing; The spacers areslight; 1y thicker than the "IGSiStOI flQKeS 50.131131}? the flakes will be allowed: to' shrink-: during-.firin Withoutmechanicalconstraint;

The" stacks of loaded carrier platesz. mayf'be placed in a "suitable supportforisuspension:inla furnace. For example. as illustrated inzFigri-2; a plurality ofsupporting plates .Hli'ea'ch. loaded with one or more flakes II andiseparatedi'from adjacent plates by spacers I21 may.-:be-mounted on the support or hanger [3. The support may be simple U-shaped elements of? wire having overturned upper ends. two: orimorezofr: which may be used to: support:a.stack,.thexfrontr one only being shown iniig; 2. If desired?cross.=bars may be employed to connect two or: morefhangers together into a rack-.1

A" rack of loaded plates maythen'beplacedinza suitable furnace for removaLof the temporary binder and sintering. Where the binder "is poly vinyl butyral, the'temperature'may beigradually raised to about 600 degrees'C; todepolymerize' and drive off the binder; Otheritechniques may be employed for removing. the temporary :binder; depending on its characteristics; Usuallythey will includengentle heating.

After the binder is removed. thetemperature may be raised rapidly to about. :1100 degrees LC.

. Mostof the sintering.:of the flakestakesiplace in this" range; This: .1 temperature; may be: further raised to 1 some :pointtbetweernllOQ and11450 'de& grees C. to complete:the..sinteringidependingon the material from which the flakes aremade; The'fiakes and their support arethen allowed to cool and'the flakes areremoved from the eupporting plate.

The completed flake Will then appear as disclosed'in Fig; 8 and will comprise a-layercf ther mistor material it! and a layer ofelectrodematerial l5 intimately joined to both its major'surfaces: For some applications it maybe desirable to "employ a thermistor of this type'with athermai sink. In such a case it is difficult to flnda material of permissive cost having the necessary thermal properties and expansion characteristics such that the'thermistcr'can beattached directly thereto" without either, destroying the thermistor or breaking-it away from the surface of the. sinktas it is 'subjected'to large temperaturech'anges: A

mounting which is not subject to such expansion limitations is disclosed in Fig. 4. It comprises a strip of metal I6, having a similar coefficient of expansion to that of the thermistor layer is and its electrode [5, to which the flake electrode is is directly secured, as by welding. This strip, which may conveniently be of platinum ribbon where the electrode is of platinum, is then secured to a sink, which may comprise a block I l of some material having a high thermal diffusivity such as copper, so that the portion of the platinum ribbon supporting the thermistor is spaced therefrom. A slot IS in the face of the block under the thermistor will achieve this result. Thus the platinum ribbon, being in intimate contact with a large area of both the block and the thermistor, and having a high thermal conductivity, readily transmits the heat of the thermistor to the sink while providing one conductive path to the lower electrode IS. The conductive path may be completed through the thermistor and upper electrode !5 by securing a contact to the upper electrode as by welding another platinum ribbon thereto or by employing a method hereinafter set forth in regard to the construction of the thermistor of Fig. 5.

Fig. 5 illustrates a flake thermistor with wire leads attached. Difficulty has been encountered in attaching leads to semiconductive material as there is a tendency for a change in composition to occur and thereby change the electrical characteristics of the device when heat is applied thereto. These diificulties, which might also occur in some metal-thermistor mixture electrodes, can be avoided in the case of flake thermistors manufactured according to this invention by modifying the production techniques set forth above so that where a metal-thermistor material electrode layer I9 is employed an additional layer 29 substantially entirely of metal is applied in the formation of the film. This is done by first applyin a thin layer of the all-metal paste to the optical flat, then the metal-thermistor material, thermistor material, and metal-thermistor material layers, and finally applying a final layer of the allmetal paste and heat treating in the usual manner to drive all the binder and sinter the layers. Leads 2| can be attached to the resulting flake by soldering them directly to the outer all-metal layers. This eliminates the problems which are involved when leads are secured directly to thermistor material or a mixtur containing such material while requiring no further heat treatment and retaining the reduction of barrier layer eifects which are obtained when the metal-thermistor electrode mixture is used.

As an example of a thermistor made up in the form disclosed in Fig. 5, the contact layers 29 might be formed from a paste of platinum black, isobutyl methacrylate and a volatile solvent, the electrode layers 1 9 might be formed from a paste comprising a mixture of 75 per cent by volume of platinum black and per cent by volume of a thermistor material mixed with a copolymer butadine styrene and cyclohexane as a solvent. and the thermistor layer formed from a paste of thermistor material, isobutyl methacrylate and a volatile solvent. Thus the adjacent layers in the unfired thermistor have binders and solvents which are mutually insoluble and therefore there is no tendency to disrupt the desired organization thereof. The thermistor is completed by securing leads 2| to the substantially pure platinum contact layer 20 by beads 22 of silver solder.

While the precedin disclosure have included 6 composite films and resistor flakes having both major surfaces of the resistance material layer coated with an electrode layer, and while specific materials have been suggested for this resistor and its method of production, it is to be understood that this invention is not to be so limited and that one surface or only portions of one or both surfaces of the resistor layer may be coated with an electrode material of a variety of compositions.

What is claimed is:

1. The method of making a rapid response resistor having a high factor of resistance dependence upon temperatur which comprises mixing finely divided metal with a temporary binder, spreading a film of the mixture on a surface, mixing finely divided high resistance-temperature coefiicient semiconductive material with a temporary binder, spreading a thin film of this mixture on the metallic mixture film, removin the sheet comprising the superposed films from the surface, and heat treating the sheet to remove the temporary binder from the layer and to consolidate the finely divided material into a self-sustaining resistor.

2. The method of making a rapid response resistor having a high factor of resistance dependence upon temperature which comprises mixing finely divided metal with a temporary binder, spreading a film of the mixture on a surface, mixing finely divided high resistance-temperature coefiicient semioonductive material with a temporary binder, spreading a thin film of this mixture on the metallic mixture film, spreading another metallic mixture film on the semiconductive mixture film, removing the sheet comprising the superposed films from the surface, and heat treating the sheet to remove the temporary binder from the layers and to consolidate the finely (iivided material into a self-sustaining resistor.

3. The method of making a rapid response resistor having a high factor of resistance dependence upon temperature which comprises mixing finely divided platinum black with a temporary binder, spreading a thin film of the mixture on a smooth surface, mixing finely divided highresistance-temperature coefficient semiconductive material with a temporary binder, spreading a thin filn of this mixture on the platinum mixture film, spreading another platinum mixture film on the semiconductive mixture film, drying the sheet comprising the superposed films, removin the sheet from the surface, dividing the sheet into flakes, and heat treating the flakes first at a moderate temperature to remove the empor y binder and then at a higher temperature to consolidate the finely divided material into a selfsustaining resistor.

4. The method of making a rapid response resistor having a high factor of resistance dependence upon temperature which comprises mixing finely divided high resistance-temperature coefficient semiconductive material and finely divided metal with a temporary binder, spreading a film of the mixture on a surface, mixing finely divided high resistance-temperature coefiicient semiconductive material with a temporary binder, spreading a thin film of this mixture on the semiconduotive material-metal mixture film, spreading another semiconductive material-metal mixture film on said semiconductive mixture fi1m,'removing the sheet comprising the superposed films from the surface, and heat treating the sheet to're'move the temporary binder from the layers and to consolidate the finely divided; material; into a selfesustaining. resistor;

5. Thamethodzof: making a rap d response resistor having. a. .high;.factor-- of. :resistance dependence upon: temperature, which comprises mixing finely divided high resistance-temperaturecoefiicient'semiconductive material and finely divided platinum black. with a temporary binder, spreading the; film of the; mixture. on a surface, mixing finely dividedhigh resistancetemperature coefficientisemiconductive material with a temporaryzbindeh; spreading a thinfilm of this mixture on .theasemiconductive materialplatinum,mixturefilm, spreading a semiconducv tive-materialplatinum;mixture film on theIsemiconductivemixture-film, removingthe sheet com-. prising the superposedgfilms. from-;the;.-surface,.

and heat treating thesheet to removeetheteme por-ary binder from;thelayers and .to consolidate the finely dividedmaterial intoa self-sustaining resistor.

6. The methodof makingxa rapid response. resistor having a high factor of resistance, de-- pendence upon temperature which comprises mixing finely. divided high resistance-temperature coefficient semiconductive material and finely divided metal with-atemporary binder, spreading a film of .the' mixture on a smooth, plane surface, mixing. finelyv divided high resistancetemperature coefficient: semiconductive material with a temporarybindergrspreading a thin film of this mixture on. the semiconductive materialmetal mixture film, removing the sheet comprising the superposed films from the plane surface, and heat treating the'sheet to remove the temporary binder from the layers and to consolidate the finelydivided materialinto a self-sustaining resistor.

7. The method of making arapid response resistor having a high factor:;of resistance dependence upon temperature which comp-rises mixing finely divided platinum. black with a temporary binder, spreading a thin film of the mixture on a smooth, plane surface, mixing finely divided high resistance-temperature coefficient semiconductive material and finely divided platinum black with a temporary-binder, spreading a thin film of this mixture on the platinum mixture film, mixing finelydivided' high resistancetemperature coefiicient semiconductive material with a temporary binder, spreading a thin film of this mixture on the platinum-semiconductive material'mixture film, spreading. another plati-- num-semiconductive material mixture film on thesemiconductive film, spreading another platinum; film" on, the. platinumesemiconductive ,ma-r rial mixture film, drying the-sheetcomprising, the superposedfilms,: removing the sheet from the plane. surface, dividing thesheet into small. pieces, andheathtreating the. pieces first at a moderate temperature to remove the temporary binder andthen ate-high temperature to consolidate the finely divided materialinto a selfsustaining resistor.

8. The method. of making resistor elements thatcomprisesmixing finely dividedmetal particles. with a temporary binder, applying a thin film of the mixtureto a smooth; flat plate, mixing finely divided resistance" material with a temporary binder, spreading a' film of the'resistance material 'on; said metal. mixture film, removing the sheet comprising the superposed films from the plane; surface, and' heat' treating; thesheet to remove; the-temporary binder-and consolidate 8? therfinel-y dividedmaterial into a, self-sustaining. resistor.

9. The: method of making. resistor. elements. that comprises mixing finelydivided resistance material and finely dividedmetal particleswith. a temporary binder, applying a thin film of. the mixture to a smooth, fiat plate, mixing finely. divided resistance material with a temporary binder, spreading a film of the resistance. ma-. terial on the resistanceemetal mixture film, removing thesheet comprising the superposed. films from the plane surface, and heat treating the sheet to remove-the temporary binder from the layers and to consolidate the finely divided material into a self-sustaining resistor.

10. Themethod of making resistonelements that comprises mixing finely divided metal-with. a temporarybinder, spreading a film of the mixture on a smooth, plane surface, mixing a-finely divided resistor material with a temporary binder, said binders being mutually insoluble, spreading; a film of the resistor material on said: metal. mixture film, drying the sheet-comprising.-the-. superposed films, removing the sheet fromthe plane surface, and-heat treating the sheet to insoluble with that of the metal-resistor mixture material, spreading a film of the resistor material on said metal-resistor mixture film, drying the sheet comprising the superposed films, removing the sheet from the plane surface, and heat treating the sheet to remove the temporary binder and to consolidate the finely divided materialinto a self-sustaining resistor.

12; The method of making-thin flake resistor elements which comprises laying down a plurality of superposed fluid films includinganelectrode film and an adjacent resistor film on a surface, drying each film prior to laying the next film on. it, dividing the flake composed of the: dried superposed films into sections of the.de sired. shape and area, and heat treating the sections to form them into self-sustaining re,- sistor elements. 1

' JOSEPH A. BECKER.

HOWARD CHRISTENSEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS.

Number Name Date 2,106,249 Hower Jan. 25, 1938 2,407,251 Christensen Sept. 10; 1946 2,496,346" Haayman etal. .Feb. .7, 1950 FOREIGN PATENTS Number Country Date I 115,195- Australia .June- 4, 1942, 

1. THE METHOD OF MAKING A RAPID RESPONSE RESISTOR HAVING A HIGH FACTOR OF RESISTANCE DEPENDENCE UPON TEMPERATURE WHICH COMPRISES MIXING FINELY DIVIDED METAL WITH A TEMPORARY BINDER, SPREADING A FILM OF THE MIXTURE ON A SURFACE, MIXING FINELY DIVIDED HIGH RESISTANCE-TEMPERATURE COEFFICIENT SEMICONDUCTIVE MATERIAL WITH A TEMPORARY BINDER, SPREADING A THIN FILM OF THIS MIXTURE ON THE METALLIC MIXTURE FILM, REMOVING THE SHEET COMPRISING THE SUPERPOSED FILMS FROM THE SURFACE, AND HEAT TREATING THE SHEET TO REMOVE THE TEMPORARY BINDER FROM THE LAYERS AND TO CONSOLIDATE THE FINELY DIVIDED MATERIAL INTO A SELF-SUSTAINING RESISTOR. 